Facilitating catch-up growth and preventing growth faltering is a major component of clinical care for children born preterm to at least age two. However, our preliminary data suggest that obesity is a major, under- recognized problem in the preterm population and likely emerges differently than for children born at term. Obesity has harmful consequences for children born preterm, far greater than the consequences for children born at term. Without knowledge to fill gaps about the extent, nature, and timing of obesity development and how consistently obesity is recognized in clinical care, tailored prevention strategies cannot be developed to help clinicians and families promote adequate growth while preventing excess growth among preterm children. The long-term goal, which will be the subject of a subsequent R01 application, is to develop evidence-based strategies tailored to preterm children to guide clinicians and families in preventing overweight and obesity. The objective of the study proposed in this R03 application, which is the next step towards that goal, is to apply cutting edge statistical methods to determine the prevalence and timing of the development of overweight and obesity in early childhood among preterm children, as compared to term children, to identify optimal intervention windows, and to document the extent of under-recognition of obesity during clinical care in a longitudinal sample of more than 16,000 children. The central hypothesis is: as compared to children born at term, growth trajectories will differ most especially before age 3 among preterm children but overweight and obesity will be nearly as prevalent by ages 4-6, and that obese children born preterm will be less likely to have an obesity diagnosis. The hypothesis was formulated based on data from historic low birth weight cohorts and our preliminary data. The rationale for the proposed study is to support the development of evidence-based interventions targeting sensitive periods to prevent overweight and obesity in children born preterm. The hypothesis will be tested by pursuing two specific aims: 1) Identify critical windows for obesity prevention by characterizing growth trajectories from birth to age 6 for children born preterm, and contrasting these with trajectories for children born at term, to compare the prevalence and timing of the development of overweight and obesity, and 2) Determine the extent of obesity under-diagnosis among children born preterm, as compared to children born at term. The approach will be a longitudinal study of more than 16,000 children from birth to age 6, patients of primary and subspecialty care providers at a major free-standing children's hospital. The proposed research is innovative because it will apply state-of-the-art analytic tools to model growth trajectories in a modern sample, using a detailed, research-ready longitudinal growth dataset based in a very large pediatric patient population, including children born preterm and at term, with an average of more than 13 measures before age six. The proposed research is significant because it is the next step in devising strategies to help clinicians and families promote healthy growth in children born preterm while preventing obesity.